1. Field of the Invention
The present invention relates to magnetoresistive sensors for reading magnetically recorded information from data storage media, and particularly to spin valve read sensors for direct access storage device (DASD) systems.
2. Description of the Prior Art
By way of background, spin valve sensors, also known as giant magnetoresistive (GMR) sensors, are commonly used in read heads for magnetic media-based DASD systems, such as disk drives. A spin valve sensor is a magneto-electrical device that produces a variable voltage output in response to magnetic field fluctuations on an adjacent magnetic storage medium. As illustrated in FIG. 1, a conventional spin valve device is formed by first and second ferromagnetic layers, hereinafter referred to as a “pinned” layer and a “free” layer, separated by an electrically conductive spacer layer. In a disk drive, these layers are oriented so that one edge of the layer stack faces an adjacent disk surface, in a cross-track direction, and so that the layer planes of the stack are perpendicular to the disk surface. The magnetic moment (M1) of the pinned layer is oriented at an angle θ1 that is perpendicular to the disk surface (i.e., θ1=90°). It is sometimes referred to as the “transverse” magnetic moment of the sensor. The magnetic moment M1 is substantially pinned so that it will not rotate under the influence of the disk's magnetic domains. Pinning is typically achieved by way of exchange coupling using an adjacent antiferromagnetic pinning layer. The magnetic moment (M2) of the free layer has a zero bias point orientation θ2 that is parallel to the disk surface (i.e., θ2=0°). It is sometimes referred to as the “longitudinal” magnetic moment of the sensor. The magnetic moment M2 is free to rotate in positive and negative directions relative to the zero bias point position when influenced by positive and negative magnetic domains recorded on the disk surface. In a digital recording scheme, the positive and negative magnetic domains correspond to digital “1s” and “0s.” The zero bias point is the position of the free layer magnetic moment M2 when the sensor is in a quiescent state and no external magnetic fields are present.
Electrical leads are positioned to make electrical contact with the pinned, free and spacer layers. In a CIP (Current-In-Plane) spin valve sensor, as shown in FIG. 1, the leads are arranged so that electrical current passes through the sensor stack in a cross-track direction parallel to the layer planes of the stack. When a sense current is applied by the leads, a readback signal is generated in the drive processing circuitry which is a function of the resistance changes that result when the free layer magnetic moment M2 rotates relative to the pinned layer magnetic moment M1 under the influence of the recorded magnetic domains. These resistance changes are due to increases/decreases in the spin-dependent scattering of electrons at the interfaces of the spacer layer and the free and pinned layers as the free layer's magnetic moment M2 rotates relative to the magnetic moment M1 of the pinned layer. Resistance is lowest when the free and pinned layer magnetic moments are parallel to each other (i.e., θ2=90°) and highest when the magnetic moments are antiparallel (i.e., θ2=−90°). The applicable relationship is as follows:                ΔR∝cos(θ1−θ2)∝sin θ2. The ΔR resistance changes cause potential differences that are processed as read signals.        
It is important that a spin valve sensor exhibits high GMR effect ratio (i.e., a high ratio of change in resistance to the resistance of the sensor as a function of an applied magnetic field) in order to provide maximum sensitivity. It is likewise desirable to construct the free layer so that it exhibits controlled negative magnetostriction for high stability.
As the areal density in magnetic recording increases, it is necessary to reduce the magnetic thickness of both the recording medium and the free layer of the sensor. The magnetic thickness of a material is given by the product of the remanent magnetic moment density (Mr) and physical thickness (t) of the material, and is commonly expressed as Mr*t. The conventional approach to decreasing the free layer magnetic thickness in a spin valve sensor is to decrease the free layer's physical thickness, e.g., from 30 Å to 25 Å or below. Unfortunately, reducing the free layer's physical thickness tends to decrease sensor sensitivity by reducing its GMR ratio and causing free layer magnetostriction to become more positive.
One approach to improving the performance of spin valve sensors with thin free layers is a “spin filter” design, in which a thin layer of highly electrically conductive and non-magnetic material, typically copper (Cu), is inserted between the sensor free layer and its (preferably oxide) cap layer. With this design, however, the spin valve sheet resistance undesirably decreases.
Accordingly, a need exists for a GMR sensor configuration wherein decreased free layer magnetic thickness is achieved in order to accommodate increased areal data densities while maintaining high sensor GMR ratio and controlled negative magnetostriction in the free layer. What is required in particular is a GMR sensor having a free layer with decreased magnetic thickness and improved sensitivity without having to decrease free layer physical thickness and thereby negatively impact sensor GMR ratio and free layer magnetostriction.